Centre every object in relation to each other
Either select all objects, center the cursor with Shift + C and then use Shift + S > Selection to Cursor (offset); or if that doesn't work:


Select all objects with A
Clear their relations by removing parenting with Alt + P
Center objects origins with Shift + Ctrl + Alt + C > Origin to geometry
Now select all objects again with A and then Shift Select one last large object (like the ground or the scene background or whichever you want to be at the exact center of the world) by clicking on it to make it the active object
Press Ctrl + P to parent all other objects to that one
Now you can move that active "main" object to the scene's origin with whichever method you like more without breaking objects relative positions.


Either With Alt + G or with Shif + S > Object to origin or by manually entering or clearing coordinates.

You can then release the parenting if you no longer need it afterwards.

Edit: Have in mind that this may break any meaningful relationships you might previously have between your objects

P.S. Offtopic: Kudos for having Vivaldi browser installed ;)